Many portable communication devices (e.g., selective call receivers), require operation from very low power sources (e.g. a AA battery). A sensor within the device may be used to indicate when the incoming voltage potential is above a desired operating threshold. Below the operating threshold the device's circuits typically are maintained in a known standby state.
Conventional sensors may comprise a differential voltage comparator circuit, such as shown in FIG. 1. The differential voltage comparator of FIG. 1 comprises a differential pair of N-channel metaloxide semiconductor (MOS) transistors (102 and 104), having their sources (106 and 108) electrically coupled together. Additionally, a bias network comprises an N-channel MOS transistor 110 with its source 112 tied to ground. A bias voltage (Vb) applied to a gate 114 of the transistor 110 regulates a bias current at the summing point (sources (106 and 108)) of the differential pair and a drain 116 of the biasing transistor 110.
The differential pair (102 and 104) are also electrically coupled at their respective drains (118 and 120) to a loading network that comprises two P-channel MOS transistors (122 and 124) in a conventional "current mirror" configuration. As is known, the loading network regulates a load current supplied to the differential pair (102 and 104).
Two respective input voltage potentials (V1 and V2) may be presented at the gates (126 and 128) of the differential pair (102 and 104). An output current (Iout) taken typically at a drain 120 of one of the differential pair (102 and 104) represents the compared differential input voltage potentials (V1 and V2). To determine the threshold criteria one of the input voltages (e.g., V2), may be set to a predetermined reference voltage potential. Therefore, when the other differential input voltage potential (e.g., V1), crosses the predetermined reference threshold the output signal (Iout) indicates a changed condition.
Regrettably, conventional device arrangements such as shown in FIG. 1 may exceed the desired current specifications for very low power sensor applications, as required in many portable communication devices (e.g., selective call receivers). Accordingly, a need exists for a low power voltage sensor.